About ENSO Outlooks

Explanation

Warm and cool conditions

The most common definitions of El
Niño
and La
Niña
refer to changes in sea surface temperatures (SSTs) over the tropical
Pacific Ocean. Warm conditions (El Niño)
mean that SSTs are significantly warmer than their climatological average
values for a given time of year, while cool
conditions (La Niña) mean that SSTs are colder than normal.
Generally, warm conditions correspond to negative values of the
SOI and in eastern
Australia usually to dry conditions. Cool conditions tend to correspond
to positive values of the SOI and in eastern Australia usually to wet conditions.

NINO3.4 and other indices

The National Climate Centre (NCC) uses the "NINO3.4 index" to classify ENSO
conditions (see "Note:" below). The NINO3.4 index is defined as the average of
SST
anomalies
over the region 5°N - 5°S and 170° - 120°W. NCC classifies
the NINO3.4 temperature anomaly as "warm" if it exceeds 0.8°C,
which is about one standard deviation above average. Similarly, anomaly
predictions below –0.8°C are tabled as "cool", with those in
between classed "neutral". There are also
other "NINO"
indices that refer to SST anomalies over different areas of the
Pacific Ocean. The regions covered by the NINO indices are shown in this map of the tropical Pacific Ocean.

Outlook Information

The main variable that is considered from the coupled climate models'
ENSO forecasts is the NINO3.4 index. All models that NCC surveys run a
suite of forecasts known as an ensemble. Both the ensemble mean (the
average of these forecasts) and the individual forecasts are useful:
the spread amongst individual forecasts indicates how sensitive the
forecasts are to the exact initial conditions from which they are run.
Note that the temperature conditions cannot always be determined
precisely from published materials and then a subjective assessment is
made from available material. This material is linked to the
institutions listed below. The information given here is intended only
as a quick summary of forecasts. For more detail visitors are advised
to visit the linked websites.

The fraction or proportion of models categorized as warm, neutral or
cold, should not be taken as an official Bureau or National Climate
Centre view as to the likelihood of these various outcomes.

Prediction Systems

Nearly all the forecast systems whose results are included in the ENSO
forecast have been documented in the reputable peer-reviewed
international scientific literature and have a respectable level of
skill in predicting ENSO. They are all coupled General Circulation
Models, so they simulate both atmospheric and ocean processes and are
dynamic rather than statistical models. Most have been run routinely for
a number of years.
All models use an ensemble method, where several model forecasts are run at the same time with minor tweaks to the initial conditions and the model parameters. Each line in a plume graph represents a model run (or ensemble member) with the number of ensemble members included varying from model to model.

Some of the models are regarded as experimental, and most are subject to
regular revision and update with the aim of continuous improvement.
However, they represent the state-of-the-art as far as dynamical ENSO
prediction systems are concerned and offer useful guidance up to about
nine months.

Note: Until recently NCC used NINO3 index to monitor ENSO conditions.
However, NINO3.4 is now used since this has been shown to be more closely
related to Australian climate than NINO3. See, for example,
Wang, G. and Hendon, H. H., 2007: Sensitivity of Australian rainfall to
inter-El Niño variations. Journal of Climate, 20,
4211-4226.